In both natural and technological contexts, the degree of saturation with respect to water often plays a central role in defining a system's properties and function. For example, in the atmosphere, relative humidity is a critical meteorological indicator, and is important to evaporative demand on soil, bodies of water, and the biosphere. In the context of plants and agriculture, water saturation in the soil and atmosphere controls viability, growth potential, yield, and quality of crop. In foods, water activity affects taste, texture, and stability with respect to bacterial and fungal growth. In chemical and biological processes, the osmotic strength of aqueous solutions controls the kinetics and thermodynamics of reactions and the stability of cells, proteins, and materials. Additionally, the water status and dynamics of water in concrete is critical to final quality.
The chemical potential of water, μw [J mol−1], within a phase or host material provides the most generally useful measure of the degree of hydration. This thermodynamic state variable quantifies the free energy of water molecules and thus their accessibility for chemical reactions and physical exchange with other phases or materials. For example, regardless of the local mode of transport, the driving force for mass transfer can be expressed as a gradient of chemical potential. The chemical potential of water can be characterized with two convenient state variables. The first is activity, aw, defined as the relative humidity of a vapor in equilibrium with the phase of interest (aw=p/psat(T), where p and psat(T) are the vapor pressure and saturation vapor pressure at temperature T, respectively). The second is water potential, Ψw [MPa], the deviation of the chemical potential from its value at saturation divided by the molar volume of liquid water (Ψw=μw−μ0(T))/vw,liq). Water potential is widely used in the plant and soil science communities. The typical water potential range of plants and soils is −0.001>Ψw>−3.0 MPa (0.99999>aw>0.978).
For in situ measurements, existing methods of hygrometry include capacitance, resistance, thermal conductivity, psychrometric, and tensiometric. Capacitance, resistance, and dielectric methods measure the corresponding electronic property of a calibrated material within the sensor that is allowed to reach its equilibrium hydration with the phase of interest. Although these methods allow for small form factors (e.g., <1 cm2 sensing areas), they suffer drawbacks. One drawback is that they generally provide moderate-to-low accuracy for drier conditions (i.e., ±˜0.02 in activity for aw<0.9; ±˜3 MPa in water potential). Another drawback is they become less accurate above this range (i.e., ±25% of measurement of water potential for the MPS-2 dielectric hygrometer by Decagon), and the response time 10-60 minutes. Despite their limited accuracy, resistive and capacitive sensors are widely used for coarse measurements of water status in soils for irrigation scheduling.
Psychrometry, and thermocouple psychrometry in particular, has been heavily studied for in situ hygrometry in the environmental context. Thermocouple psychrometry involves the measurement of the dew point temperature on a wetted thermocouple evaporating into a volume of air that separates it from the sample of interest. It is a transient, non-equilibrium process. The range of commercial psychrometers is reported by the manufacturer to be 0.999 to 0.93 in activity and −0.1 to −10 MPa in water potential with an accuracy of ±0.001 in activity and ±0.1 MPa in water potential. These devices have good response time (˜1 min.), however, they are temperature-sensitive and expertise is required for installation.
Tensiometers operate on the principle of equilibration between a sample of interest and an internal volume of liquid water via a vapor gap and a porous membrane. Commercially-available tensiometers consist of an air-tight, water-filled tube with a porous ceramic tip at the bottom and a vacuum gauge at the top. The tensiometer is partly buried in the soil to a suitable depth, and the ceramic tip allows water to move freely in or out of the tube. As the soil dries out, water is sucked out through the porous ceramic tip, reducing the pressure inside the tensiometer to values below atmospheric pressure; this pressure is read on the vacuum gauge. When the soil is wetted by sufficient rainfall or irrigation, water flows back into the tensiometer, the pressure rises and the gauge reading rises.
Commercially-available tensiometers have a small range of 1 to 0.9988 in activity or 0 to −0.16 MPa in water potential with an excellent accuracy of ±5×10−4 MPa in water potential. However, they have a long response time (˜30 min.) and they fail due to invasion of air or cavitation beyond this range. Despite the extremely limited range and large form factors of conventional tensiometers (sensing area>10 cm2), their unmatched accuracy near saturation means that they are used extensively to monitor the water potential in soils for irrigation scheduling for amoral crops that require moist conditions to grow.
Current research efforts have pursued two strategies to extend the operational range of tensiometers. The first strategy used porous membranes with smaller pore sizes to achieve stability out to Ψw=−1.5 MPa (aw≅0.99). However, these so-called “high capacitance tensiometers” have similar form factors as those of conventional tensiometers. The second strategy used osmotic solutions within the internal volume of the tensiometers to extend the stability limit. This approach has been refined and demonstrated out to Ψw=−1.6 MPa (aw=0.988) with a reduced form factor (1.5 cm2).
Furthermore, local water content and chemical potential define the physical properties of materials, the rates of chemical transformations, and the accessibility of water for exchange within the local environment. Fluxes of water are strongly coupled to and, in many cases, control the transport of energy and other chemical species. The ability to understand and predict natural processes such as climate change and optimize human processes such as irrigation depend on the ability to measure water content, chemical potential, and flux quantitatively, with appropriate spatial and temporal resolution and accounting for their complex coupling to temperature, material properties, and biological response.